1. Field
It is related to a temperature characteristic correction circuit and a sensor amplification circuit.
2. Description of the Related Art
Miniaturization of sensors has resulted in a generation of finer output signals. Amplification circuits for detecting and amplifying such sensor outputs are widely used. An output signal from a sensor has temperature characteristics. Thus, an amplification circuit for a sensor must correct the temperature characteristics of an output signal before amplifying the output signal. In addition to parameters that just increase or decrease, temperature characteristics of the sensor includes parameters having voltage variation gradients that change in accordance with the temperature.
FIG. 1A is a schematic block circuit diagram of an amplification circuit 90 that includes a conventional temperature characteristic correction circuit 80. When an output signal Vs of a sensor (not shown) has a predetermined gradient (e.g., a V-shaped gradient as shown in FIG. 1B), the correction circuit 80 generates an adjustment signal Va (FIG. 1C) that offsets the gradient of the output signal Vs. The adjustment signal Va is provided to the sensor amplification circuit 90, which receives the output signal Vs. The amplification circuit 90 corrects the temperature characteristics of the output signal Vs using the adjustment signal Va and generates an amplified signal Vout (FIG. 1D).
The correction circuit 80 will now be described with reference to FIG. 2. The correction circuit 80 generates the adjustment signal Va having, for example, the V-shaped gradient shown in FIG. 1C.
As shown in FIG. 2, a correction amplifier 81 has two input terminals. One input terminal is provided with a temperature dependent signal Vt of which the voltage value is dependent on the temperature as shown in FIG. 3A. The other input terminal is provided with a reference signal Vref of which the voltage value is substantially the same regardless of temperature as shown in FIG. 3A. The correction amplifier 81 inverts the temperature dependent signal Vt based on the reference signal Vref and a gain that is in accordance with a resistor R11 and a feedback resistor R12 to generate a correction signal Vc. The temperature dependent signal Vt and the reference signal Vref are also provided to a comparator 82. The comparator 82 compares the temperature dependent signal Vt with the reference signal Vref and generates a selection signal SS for switching switches S1 and S2 based on the comparison result.
In detail, when the voltage value of the temperature dependent signal Vt is higher than the voltage value of the reference signal Vref (at temperatures lower than a switching temperature Ts indicated by a broken line in FIG. 3A), the comparator 82 provides the switches S1 and S2 with a high (H) level selection signal SS. The H level selection signal SS couples a first terminal all of the switch S1 to a common terminal c11 of the switch S1 and a first terminal a12 of the switch S2 to a common terminal c12 of the switch S2. As a result, the resistance of the feedback resistor R12 is set in accordance with a set value prestored in a register r1, which is coupled to the first terminal all of the switch S1. Further, the gain of the amplifier 81 is set to a predetermined value (e.g., 2). The amplifier 81 then inverts the temperature dependent signal Vt based on the reference signal Vref and the gain of 2 to generate a correction signal Vc (refer to the dashed line in FIG. 3B). The correction signal Vc is inverted by an inverter 83. The inverted signal is then provided to the sensor amplification circuit 90 as an adjustment signal Va (refer to the solid line in FIG. 3B).
When the voltage value of the temperature dependent signal Vt is lower than the voltage value of the reference signal Vref (at temperatures higher than the switching temperature Ts), the comparator 82 provides the switches S1 and S2 with a low (L) level selection signal SS. The L level selection signal SS couples a second terminal b11 of the switch S1 to the common terminal c11 and a second terminal b12 of the switch S2 to the common terminal c12. As a result, the resistance of the feedback resistor R12 is set in accordance with a set value prestored in a register r2, which is coupled to the second terminal b11 of the switch S1. This changes the gain of the amplifier 81 to another value (e.g., 1). The amplifier 81 then inverts the temperature dependent signal Vt based on the reference signal Vref and the gain of 1 to generate a correction signal Vc (refer to the double-dashed line in FIG. 3B). The correction signal Vc is directly provided from the correction circuit 80 to the sensor amplification circuit 90 as an adjustment signal Va (refer to a solid line in FIG. 3B). In this manner, the temperature characteristic correction circuit 80 generates the V-shaped adjustment signal Va that has different gradients at low temperatures and high temperatures.
Such a temperature characteristic correction circuit and sensor amplification circuit are described in Japanese Laid-Open Patent Publication No. 6-307945.
The temperature characteristic correction circuit 80 switches the switches S1 and S2, or the gradient of the adjustment signal Va, with the comparator 82. More specifically, the temperature characteristic correction circuit 80 switches the gradient of the adjustment signal Va based on the comparison of the temperature dependent signal Vt and the reference signal Vref performed by the comparator 82. However, the comparator 82 is known to repeatedly generate H level outputs and L level outputs when high-frequency noise enters the comparator 82 at temperatures that are close to the switching temperature Ts. To prevent the occurrence of a phenomenon similar to oscillation in the comparator 82, the comparator 82 is provided with hysteresis characteristics. However, as shown in FIG. 3B, when the comparator 82 is provided with hysteresis characteristics, the switching point of the gradient of the adjustment signal Va becomes deviated from the switching temperature Ts by an amount corresponding to the hysteresis width H. This forms a step that interrupts the continuity of the adjustment signal. As a result, the sensitivity of the comparator is lowered by a degree corresponding to the hysteresis width H, and the correction accuracy of the temperature characteristics decreases accordingly.